Ventilation Perfusion Relationships

Question 1

What is the alveolar gas equation?

Answer 1

PAO2 = PIO2 - (PaCO2/R)

Question 2

What is the alveolar gas equitation used to calculate?

Answer 2

What inspired O2 needs to be to produce a desired alveolar and therefore arterial O2 level
(Oxygen concentration in the alveoli)

Question 3

How is PIO2 calculated?

Answer 3

(Barometric pressure - water vapor pressure) xFiO2

Question 4

What is barometric (atm) pressure?

Answer 4

760 mmHg at sea level

Decreases with increasing altitude

Question 5

What is water vapor pressure?

Answer 5

Air is humidified as you inhale it

At body temperature (or anything close) it is 47mmHg

Question 6

What is the FiO2?

Answer 6

Fraction of inspired oxygen (percent oxygen inhaled)
21% is normal (regardless of altitude)
Changes if you supplement oxygen (ex. Pt is being given 100% oxygen)

Question 7

What is the FiO2 if the pt is breathing “room air”

Answer 7

21%

Question 8

How do you determine the health of the alveoli?

Answer 8

Calculate the A-a O2 gradient

Question 9

How is the A-a O2 gradient calculated?

Answer 9

PAO2 - PaO2

Question 10

What is the normal value for the A-a O2 gradient?

Answer 10

Anything less than 20 mmHg is considered normal

Depends on age

Question 11

An increase in the A-a O2 gradient indicates what?

Answer 11

A diffusion impairment (something is wrong at the alveoli)

Question 12

What does it mean if the A-a gradient is normal but the PaO2 and PAO2 are both low?

Answer 12

The problem is elsewhere - not in the alveoli

e.g. pt may be hypoventilating

Question 13

The intrapleural fluid is also acted on by what?

Answer 13

Gravity and affects the intrapleural pressure

Question 14

What is the Pip at the apex of the lung?

Answer 14

There is less intrapleural fluid so the Pip is more negative
Alveoli are large at rest
Alveoli can only get a little larger with inspiration

Question 15

What is the Pip at the base of the lung?

Answer 15

There is more intrapleural fluid so the pressure is less negative
Alveoli are small at rest (not as expanded by the pressure)
Alveoli can get much larger with inspiration

Question 16

What is the ventilation/perfusion (V/Q) ratio?

Answer 16

Mathematical ratio between ventilation and perfusion over the entire lung

Question 17

What is the average V/Q ratio?

Answer 17

0.8

4L/min / 5L/min

Question 18

What does a high V/Q ratio mean?

Answer 18

Ventilation is high relative to perfusion (low)

Question 19

What does a low V/Q ratio mean?

Answer 19

Ventilation is low with a high perfusion

Question 20

A low V/Q ratio represents what?

Answer 20

A shunt - based on O2 and CO2 the blood gained nothing by the lungs as if the blood never went to the lungs in the first place

Question 21

Describe a low V/Q region

Answer 21

Low PaO2
High PaCO2
Low pHa
High volume of blood relative to V

Question 22

Describe a high V/Q region

Answer 22

High PaO2
Low PaCO2
High PHA
Low volume of blood

Question 23

What is the V/Q ratio at the apex of the lungs (zone 1)?

Answer 23

High V/Q ratio leading to a higher PaO2 and a lower PaCO2
High pHa
E.g. PaO2 = 130 and PaCO2 = 28

Question 24

What is the V/Q ratio in the middle of the lungs (zone 2)?

Answer 24

About 0.8 so “normal” and our blood gases are at expected values

Question 25

What is the V/Q ratio at the base of the lung (zone 3)?

Answer 25

Low V/Q ratio
Low PaO2 and high PaCO2 with a low pHa
E.g. PaO2 = 89 and PaCO2 = 42

Question 26

What can be done to minimize the differences in the V/Q ratios in the different zones of the lungs?

Answer 26

Hypoxic vasoconstriction (constriction of pulmonary vessels to redirect blood away from hypoxic regions)